As is well known, demands for an optical storage system that can store a large amount of data, such as data for a motion picture film, have been increasing. Therefore, various types of holographic digital data storage systems incorporating therein a storage medium made of a photo-refractive crystal such as lithium niobate or the like have been recently developed for realizing high density optical storage capabilities.
The holographic digital data storage system allows a signal beam having information therein to interfere with a reference beam to generate an optical interference pattern therebetween and, then controls the optical interference pattern to be stored in the storage medium such as a photo-refractive crystal, wherein the photo-refractive crystal is a material which may react differently on optical interference patterns depending on the respective amplitudes and phases thereof. In case the incident angle, the amplitude r the phase of the reference beam is changed, hundreds or thousands of binary data may be superposedly recorded at one location on a page-by-page basis.
In the holographic digital data storage system, a signal beam is modulated with rectangular page data so that the modulated signal beam may have a shape of rectangle which is equal to that of a displaying device. However, a Gaussian distribution of a circular laser beam may cause the rectangular page data to be deformed. In such event, a peripheral region of the rectangular page data may be considerably damaged. Although circular page data may be used to modulate the signal beam, a rectangular servo pattern has been used to generate a servo signal for modifying an irradiation location of the reference beam, thereby resulting in an inefficient reproduction of the circular page data.